Here's another oldie/goldie from Jim & Phyllis Hurley:

Here is a simple empirical method which will let you know if your pond
itself is leaking.

Milk jug check to see if a pond leaks

1. Get a straight sided container (I used a milk jug with the sloping top
cut off.).
2. Put the container in the pond so that its rim sticks out above the top
of the pond.
3. Fill it with water to just about the level of the pond.
4. Turn OFF your fountains and waterfalls.
5. If your pond has a lot of shade, use two straight-sided containers, one
in the shade and one in the sun.

Now you have two bodies of water which are facing the same evaporation
conditions.

6. Measure water levels daily for a few days. If the pond is leaking, it
will fall more than the jug(s).

Milk test to locate where the leak is

1. Let your pond lose water.
1a. When the loss rate changes, you are at the level of a leak.
Record
that level.
1b. When the loss stops, you are at the level of the last leak.
2. Add about 1/4" of water to get the level just above the leak.
3. Put some milk in a spray bottle.
4. Find where you think there is a leak, or systematically work your way
around the pond edge.
5. Squirt a little milk in the water where you think the leak is.
6. If the milk simply dissipates, the leak is not there. If it is drawn
through the side, you have your leak.
7. Remember to do this at each rate-of-loss-change point, as you may have
leaks at each level.

If the pond does not leak, you will want to try out the plumbing for the
fountains.
1. If you can turn them on without spraying into the air, do so one at a
time. If you begin losing water, the plumbing for that fountain leaks.
2. If you cannot turn them on low, but can turn them on singly, try doing
so consecutively. Compare the rate of loss. Your .8 gal/minute should be
very graphic, as long as you are not losing the water by splash or mist
drifting out of the pond.
3. If you have a waterfall, run it alone. It should not be losing at ,8
gal/min.
~ jan/WA
Zone 7a